Removes the legacy analysis options override and fixes all resulting issues. This is a combination of dart fix and manual changes (mostly mechanical, but some small restructuring to address warnings more cleanly, such as creating typed structs from args when they are used repeatedly to avoid repeated casting, or making things that were unnecessarily public private).
One small opportunistic extra cleanup is that the handling of null-safety prerelease versions is removed, as any new plugin would be written null-safe from the start, so we no longer need to allow those versions.
Part of flutter/flutter#76229
This change enables the integration_tests of the following packages to run in Cirrus CI:
* google_sign_in_web
* connectivity_for_web
* google_maps_flutter_web
* url_launcher_web
Standardizes all first-party copyrights on a single year, as is done in flutter/flutter and flutter/engine. All code now uses 2013, which is the earliest year that was in any existing copyright notice.
The script checks now enforce the exact format of first-party licenses and copyrights.
Fixesflutter/flutter#78448
In all copyright messages (and in the Xcode project organization name) standardize on "The Flutter Authors", adding "The Chromium Authors" to the Flutter AUTHORS list. This reduces inconsistency in the copyright lines in this repository, moving closer to a single consistent copyright+license (as in flutter/engine and flutter/flutter)
Updates the validation script to no longer accept "The Chromium Authors" or "the Chromium project authors" in first-party code.
Adds a new CI check that all code files have a copyright+license block (and that it's one we are expecting to see).
Fixes the ~350 files (!) that did not have them. This includes all of the files in the .../example/ directories, following the example of flutter/flutter. (This does mean some manual intervention will be needed when generating new example directories in the future, but it's one-time per example.)
Also standardized some variants that used different line breaks than most of the rest of the repo (likely added since I standardized them all a while ago, but didn't add a check for at the time to enforce going forward), to simplify the checks.
Fixesflutter/flutter#77114
